The present invention relates to a boring tool, particularly to a reboring tool for enlarging the bores of workpieces.
East Germany Pat. No. 154,072 discloses such a reboring tool, comprising a cutter, carried by a carrier, said cutter having cutting-edges at its forward face end and said carrier being composed of two shanks, wherein said cutter is received at an intermediate portion, at which both shanks are joined. The first shank is of smaller diameter and extends through the bore, which is to be enlarged, whilst the first shank and certain guiding rulers of the cutter are received in guiding sleeves, between which the workpiece is held. The second shank is of the BTA (British Trepanning Association) type, which means that it is formed by two tubes, of which the first one has a smaller diameter and is placed within the second one such that a certain space is obtained between both tubes. At the joinder of this second shank, which has the double-tube structure, with the cutter, there are first openings to connect the space between the tubes with the bore and second openings to connect the bore with the interior of the inner tube. The space between both tubes is used to supply a lubricant and cooling medium to the boring spot, whilst to the interior of the inner tube the fluid is carried back, taking with it the shavings and chips, which are taken off the bore during the boring operation. In order to guarantee this flow of the medium, which is necessarily associated with that type of tool, the workpiece is closely held between both guiding sleeves, because any space between the workpiece and the sleeve, would allow the medium to leave through the same and thereby would be detrimental to the proper functioning of this type of tool. Additionally, the structure disclosed in said East German patent has the disadvantage that the shavings or chips, removed by the cutting edges of the cutter during boring operation, tend to be collected and clog in the space between the first shank and the bore, through which the first shank extends; that is for the reason mentioned, namely that no flow through this space can be allowed, because it would not be feasible with this type of boring process or tool.
Another drawback of the patented tool is its complexity. The drilling head is secured to the outer pipe of the shank by a bayonet lock and the coupling pin which establishes the connection between the pilot and the shank is threadedly connected with the drilling head. Thus, the drilling head is disposed between the threads of the coupling pin and the bayonet lock and, consequently, is called upon to take up all of the mechanical stresses which arise in the connection between the pilot and the shank. The connection between the shank and the pilot must be very strong in order to warrant the provision of spaced guiding sleeves for the pilot and the shank, i.e., to prevent undue flexing of the tool between such guiding sleeves.